Translate AI docs updates and fix assistant API links

[mintlify-development]

Summary

Translates changes from commits since the last translation PR (#4647) into es, fr, and zh:

• ai/llmstxt.mdx: Added .well-known hosting note, HTTP headers section, .md extension on page links, and reorganized code example with separate OpenAPI/AsyncAPI sections
• ai/markdown-export.mdx: Added new "Accept header" section for programmatic Markdown fetching
• ai/skillmd.mdx: Removed "(with caching disabled)" from reverse proxy note, added "Skills discovery endpoints" section with index and individual file endpoints
• api/assistant/create-assistant-message-v2.mdx: Fixed internal link from /api-reference/ to /api/
• api/assistant/create-assistant-message.mdx: Fixed internal link from /api-reference/ to /api/

Also fixed HTML entities (') → real quotes in French translation files.

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Note

Low Risk
Low risk documentation-only updates: adds new explanatory sections and fixes internal links/typography without changing runtime behavior.

Overview
Updates Spanish, French, and Chinese AI docs to reflect new guidance around llms.txt/llms-full.txt and skill.md discovery. This adds notes about /.well-known hosting, page-level HTTP discovery headers, .md link usage in llms.txt, and documents new /.well-known/skills/ index and per-skill endpoints.

Adds an Accept header section to markdown-export docs describing programmatic Markdown retrieval, and fixes assistant API doc cross-links from /api-reference/... to /api/... (plus minor quote/entity/wording cleanups in French).

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